Every year outbreaks of cryptosporidiosis are reported in the US, mainly due to ingestion of water that has been contaminated by the chlorine-resistant pathogen Cryptosporidium. Currently, limited therapeutic agents are available for cryptosporidiosis among immunocompromised patients, which is an important concern since Cryptosporidium parvum infections in immunocompromised individuals often develop into chronic, severe cryptosporidiosis that can become life threatening. Thus there is a need for a more effective therapy and a better understanding of host immune responses to this infection. Experimental Cryptosporidium infections in mice have been used to understand how cell-mediated immunity develops and to specifically define the factors driving Th1 or Th2 responses. A major factor in resistance to C. parvum infection is the production of IFN-3. Several cytokines are inducers of IFN-3; these include IL-12, and IL-18. Both of these cytokines play a role in infection as evidenced by the fact that IL-12 knockout (KO) and IL-18 KO mice are highly susceptible to C. parvum infection. Limited information exists about IL-12 in C. parvum infections; even less is known about the role of IL-18, a cytokine produced by epithelial cells and other immune cells. We have found that IL-18 is up- regulated in response to C. parvum infection and that rIL-18 is protective, even in the absence of IL-12. It is hypothesized that IL-18 has a significant role in C. parvum infection through 1) innate responses in the epithelial cells and 2) rapidly polarizing the response to a Th1 direction by inducing IFN-3 in NK cells, dendritic cells, and intestinal intraepithelial lymphocytes (IELs). Specifically, IL-18 augments the production of S7+ IELs, a population of 'high-responder' effector cells that produce much higher amounts of IFN-3 and are highly lytic. IL-18 acts either directly on IELs or by activating dendritic cells that activate IELs. In addition, dendritic cells may be a key generator of IL-18 in the intestinal tract. We plan to identify the IL-18 expressing mucosal cell populations in the intestinal tract and determine their expression levels. We will determine the impact of IL-18 on infection using murine models of infection that discern IL-12 and IFN-3 independent and innate immune mechanisms. Specifically, we will investigate its role (IL-18) in the activation of NK, dendritic cells, and IELs and determine if these same cell types may act as both responders and generators of this cytokine. This proposal aims to define the role IL-18 plays in resistance to C. parvum and to determine its effect alone and in conjunction with other cytokines, primarily IL-12, that are key inducers of IFN-3. In immunocompromised patients, such as HIV-infected individuals, there is an increase in Th2 cytokines, which may increase patient susceptibility and/or severity of infections that require strong Th1 responses.